1. Field
The present invention relates to a method of producing a Ca—La—F based transparent ceramic that includes fluorides of calcium and lanthanum, a Ca—La—F based transparent ceramic, an optical element, an optical system, and a ceramic-forming composition.
Fluorite (CaF2) has a high Abbe's number (vd) of 95, is excellent in refractive index dispersion, and shows small dispersion of refractive index for different optical wavelength. Further, fluorite has high transmittance of light ranging from ultraviolet region to infrared region. Therefore, the fluorite is known as an excellent optical material.
It is possible to correct chromatic aberration satisfactorily by combining a convex lens made of an optical material of the above-described properties and a concave lens made of a different material. Therefore, the fluorite is frequently used in various optical systems, for example, objective lenses of microscopes (e.g., Japanese Unexamined Patent Application, First Publication, No. 2004-191933).
Single crystalline fluorite has been conventionally utilized in optical systems. On the other hand, a method of producing a fluorite ceramic as a sintered calcium fluoride is known, for example, by Japanese Unexamined Patent Application, First Publication, No. 2006-206359. Japanese Unexamined Patent Application, First Publication, No. 2006-206359 describes the production method of fluorite ceramic as follows. A suspension is obtained by a reaction of a calcium compound and a fluorine compound in a solution. Next, fine particles of calcium fluoride are produced by loading the suspension in a closed vessel and heating the suspension at a temperature of not lower than 100° C. and not higher than 300° C. A sintered body is formed by heating and sintering the calcium fluoride particles at a temperature of not lower than 700° C. and not higher than 1300° C. By heating the sintered body at a temperature of not lower than 800° C. and not higher than 1300° C. while pressurizing the sintered body by a pressure of not lower than 500Kg/cm2 and not higher than 10000 Kg/cm2 in an inert atmosphere, the sintered body becomes transparent, thereby forming a fluorite ceramic.
Since the thus produced ceramic is a dense sintered body in which occurrence of voids or the like is suppressed, it is possible to achieve excellent optical properties.
In a single crystalline fluorite which has been conventionally used in optical systems, anisotropic strain is generated due to difference in thermal expansion in different crystallographic orientation at a time of increasing a temperature of the fluorite. Therefore, imaging properties of a lens tend to deteriorate due to thermal expansion strain generated by fluctuation of ambient temperature. Further, the single crystalline fluorite is inferior in processability because cracking is easily generated by an abrupt change of temperature.
Although the fluorite has a high Abbe's number, refractive index (nd) of fluorite is a very low 1.43. Therefore, even when the fluorite ceramic is used, the optical applicability of fluorite tends to be restricted.
Although a Ca—La—F based crystal having a cubic crystal structure is known, it is difficult to produce a homogeneous Ca—La—F based crystal of high crystallinity stably. Therefore, it has been difficult to utilize the Ca—La—F based crystal as an optical material.